Complex regional pain syndrome (CRPS) is a condition of severe localized pain. We have known about this syndrome since the 19th century when it was called causalgia. It was also later called reflex sympathetic dystrophy (RSD).

A person with CRPS often has had a major stressful event like trauma, infection, sprain, emotional stress, surgery or a similar event that is very hard on the body.

The body's nervous system reacts by causing the body to keep sending pain signals to the brain. Your brain then interprets these signals as ongoing pain, which triggers a pain cycle. Most people have pain in their arms or legs but almost any body part can be affected.